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For three decades, the process industries have been using sophisticated Distributed Control Systems (DCS) or Site Control And Data Acquisition (SCADA) systems for process control. These use real-time process control graphics as the Human-Machine Interface (HMI) for the operator. But when we installed these systems, there were no available guidelines as to what constituted a “good” graphic.

Mostly for convenience, we chose a P&ID view covered in live numbers. This set in place a low-performance paradigm for HMI, and inertia has done the rest. Poorly performing HMIs have been cited as significant contributing factors to major accidents. The principles of High Performance HMI (HPHMI) provide for improved operator situation awareness, better process surveillance, better abnormal situation detection and response and reduced training time for new operators. Many major companies have HMI improvement efforts underway.

PROCESS CONTROL

Using process graphics to maximise operator effectiveness

FIGURE 1: A typical 1990s graphic screen based on a P&ID

BILL HOLLIFIELD, PRINCIPAL ALARM MANAGEMENT AND HIGH PERFORMANCE HMI CONSULTANT, PAS, INC.

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DISPLAYING INFORMATION The Figure 1 graphic does not show the operator if the process is running well or poorly. That requires specific training and months to years of experience in normal and abnormal situations.

Operators must compare each number to a memorised mental map – a difficult cognitive process. Most operators have well over a thousand such numbers

and status indications spread over dozens of graphics. Detecting abnormal conditions is difficult.

High performance displays depict information. Information is data, in context, made useful.

In Figure 2, the indicators show the process value, plus where it is compared to “good,” and to alarm and interlock ranges.

Abnormal conditions stand out clearly. Humans intuitively understand analog depictions. An operator can spot a problem with a single, two-second glance. Operators can effectively scan dozens of these values in a few seconds. This supports surveillance of the process and early detection of abnormalities. The best knowledge of desirable operating conditions is coded into the display and in view all the time. Variability in the proficiency and knowledge of individual operators is reduced. Operator training time is also significantly reduced, since important knowledge is not acquired through hit-or-miss experiences.

USE AND ABUSE OF COLOURHPHMI eliminates the common overuse and misuse of colour. Colour alone is not used as the sole discriminator of an important status condition. Use of colour is augmented by text and shape redundancies. The same colours designated for alarms must not be used for other trivial purposes.

PROCESS CONTROL

FIGURE 2: Analog depiction of information

FIGURE 3: Depicting status and alarms with redundant coding and proper colour usage

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FIGURE 4: Example level 1 display

Figure 3 indicates some poor colour coding vs. proper prac-tice. Colour is used consistently, effectively and sparingly. Bright colours are used to draw attention to abnormal situations.

GRAPHIC HIERARCHYDisplays are designed in a hierarchy providing progressive exposure of detail. A Level 1 Process Area Overview is a “big picture” showing the operator’s entire span of control. It depicts the most important information and the key performance indicators. The Overview is a good use of a large-format wall screen. Figure 4 is an Overview display of a

large power plant. At a glance, the operator can detect if the various parts of the process are running well.Every process consists of smaller, separate unit operations. A Level 2 Process Unit display (Figure 5) for each one is designed for detailed surveillance and control manipulations. It contains all the information and controls required to perform most operator tasks. A typical operator might have about a dozen Level 2 graphics.

Selecting any value or element brings up the detailed faceplate, in the reserved area, for control manipulation. Embedded trends are used in all HPHMI graphics.

FIGURE 5: Example Level 2 Display of a Reactor

PROCESS CONTROL

Level 3 Process Unit Detail graphics are used for a detailed troubleshooting. A P&ID type of depiction is often desirable. Level 4 Diagnostic Displays show detail of subsystems, individual sensors, or components.

For existing systems, most of the benefits of HPHMI can be obtained by creating about 20 new displays – typically a Level 1, a dozen or so Level 2s, and a few new Abnormal Situation displays. The existing graphics are used for Level 3. Inertia, not cost, is the primary force preventing HMI improvement.

There are dozens of additional HMI topics and examples in “The High Performance HMI Handbook,” and in a free, 60-page white paper available at PAS.com. The white paper includes detailed case studies proving the benefits of these concepts. Operators respond positively when shown examples of HPHMI concepts.

CONCLUSIONThe HMI is the primary tool for the operator to successfully run the process. Today’s sophisticated control systems are typically operated via ineffective and problematic HMIs. Operator performance can be greatly enhanced by HMIs reflecting proper principles. A High Performance HMI is practical, achievable and affordable. ■